(1) Field of the Invention
The present invention relates to a voltage controlled amplifier (VCA) and a method of using the same.
(2) Description of the Related Art
An problem that often occurs in audio amplification is clipping. Clipping occurs when an amplifier is overdriven by high input signal levels and attempts to deliver an output voltage beyond its maximum capability, as shown in FIGS. 1A and 1B. In FIG. 1A, the peaks 102 and troughs 103 of signal 101 are reproduced accurately, so no clipping occurs. In FIG. 1B, the peaks 105 and troughs 106 of signal 104 exceed the maximum output voltage 107 of the amplifier, so the amplitudes of peaks 105 and troughs 106 are clipped to the maximum output voltage 107.
Clipping can be limited with circuitry that detects clipping in an amplifier output signal (using methods well known in the prior art) and reduces the amplitude of the amplifier input signal to a level required to minimize the clipping. Examples of prior art clipping detection and reduction circuits are disclosed in U.S. Pat. Nos. 5,430,409 and 5,453,409. Input signal amplitude reduction may be accomplished with a voltage controlled amplifier (VCA) whose gain is controlled by a voltage signal received from the clip detection circuitry. Examples of prior art VCA's are the PA381 and PA382 dual low noise VCA integrated circuit devices from On-Chip Systems.
A VCA may alternatively be implemented using an Operational Transconductance Amplifier (OTA). An OTA is an amplifier whose differential input voltage produces an output current, which produces an output voltage across a resistive load. The gain (Vout/Vin) is controlled by a gain modifying input current, which can be derived from a control voltage connected across a resistor. Accordingly, an OTA can be used as a VCA. Commercial IC implementations of OTAs include the LM13700 and LM13600 devices from Texas Instruments, which each contain two OTA circuits within a single package. These commercial implementations are relatively large and, because they contain two OTA circuits, are not appropriate for applications where individual OTA circuits are needed.
FIG. 2 is a schematic diagram illustrating an example discrete implementation of a topology of an OTA circuit used in prior art commercial IC devices. The circuit of FIG. 2 includes a positive supply voltage (V+) input 201, a negative supply voltage (V−) input 202, a signal input 203, a signal output 204, ground connections 205, an operational amplifier (“op amp”) 206, a bias current input 207, resistors 208, 209, 216, 217, 218 and 219, and five matched transistor pairs 210, 212, 213, 214 and 215,
Gain for the OTA circuit of FIG. 2 is the ratio between signal output 204 and signal input 203, commonly written as Vo/Vin. The operation of the circuit is that of a normal inverting op amp circuit, where the current flowing through input resistor 208 to summing node 220 is equal to the current flowing through feedback resistor 209. For gain reduction, the current fed to summing node 220 is increased by the OTA circuit, effectively acting as a smaller value of feedback resistor 209, and thereby limiting the gain.
In the implementation of FIG. 2, the non-inverting input (+) of op amp 206 is kept essentially at ground. To achieve gain reduction of an AC input signal, the gain modifying current fed into the inverting (−) input must be bipolar. To produce the bipolar gain modifying current needed at the non-inverting input of op amp 206 requires referencing the bias current input 207 to V− and multiple pairs of matched transistors, resulting in the complexity of the circuit of FIG. 2.
The discrete OTA circuit implementation of FIG. 2, with its five matched pairs of transistors plus the op amp, is not a viable alternative to the prior art commercial OTA IC devices because it would be larger and more expensive than the commercial prior art IC devices. Thus, there is a need for a VCA circuit alternative that is less complex and less expensive than prior art commercial OTA-based VCAs and discrete implementations thereof.